1. Field of the Invention
This invention relates to an aluminum surface treatment agent, a treatment method, and to treated aluminum, and in particular, to an aluminum surface treatment agent having excellent anticorrosion properties.
2. Description of the Related Arts
Aluminum and aluminum alloys are widely used in heat exchangers because they are lightweight and have excellent workability and thermal conductivity. Today, air conditioning systems are a commonplace feature of everyday life, and the use of such systems for cooling, dehumidifying and dual function cooling and heating is increasing. The heat exchanger parts of these devices generally employ fins made of aluminum alloy.
When an air conditioner is used for cooling, moisture present in the air tends to accumulate as condensation on the fin surfaces. The fin surfaces may be made water repellent to counteract this, but this may result in water adhering as hemispherical drops spanning the spaces between fins, interfering with air intake and increasing resistance to air flow. This decreases heat exchange efficiency.
Aluminum and its alloys normally have excellent anticorrosion properties. However, condensation accumulating on fin surfaces for long periods of time may form oxygen concentration cells, while pollutants in the atmosphere gradually build up and concentrate leading to hydration reactions and corrosion. The corrosion products which may accumulate on the fin surfaces not only impair heat exchange properties, but in winter, when the devices are used for heating, they form a fine white powder which is discharged together along with warm air.
Conventionally, surface treatment agents containing chromium were used to maximize the corrosion resistance of aluminum surfaces, however as these chromium-containing agents present an environmental pollution risk, it has become common in recent years to use non-chromate surface agents. However, the anticorrosion properties of these non-chromate type agents are somewhat inferior to those of chromium-containing agents, and therefore non-chromate type surface treatment agents incorporating various improvements have been proposed.
For example, in the "Aluminum Non-Chromate Surface Treatment Agent and Treatment Method" described in U.S. Pat. No. 5,089,064, an acidic agent is proposed comprising water and (A) 0.8-1.2 (w/o) H.sub.2 ZrF.sub.6 (known as dihydrohexafluorozirconate or fluorozirconic acid), (B) 0.08-0.12 (w/o) dispersed silica, (C) 0.08-0.12 (w/o) water-soluble or water-dispersible 3-(N-methyl-N-2-hydroxyethylaminomethyl)-4-hydroxystyrene polymer, and (D) 0.10-0.15 (w/o) 1-propoxy-2-propanol.
In the "Metal Treatment Composition and Treatment Method" described in WO 9514539, a metal treatment composition is proposed comprising (A) H.sub.2 TiF.sub.6, H.sub.2 ZrF.sub.6, H.sub.2 HtF.sub.6, H.sub.2 AlF.sub.6, H.sub.2 SiF.sub.6, H.sub.2 GeF.sub.6, H.sub.2 SnF.sub.6, HBF.sub.6, (B) a water-soluble organocarboxylic acid comprising at least two OH groups (excluding the OH groups in COOH), and, if necessary, possibly further comprising (C) the elements Ti, Zr, Hf, Al, Si, Ge, Sn, B, or their oxides, hydroxides or carbonates, or (D) x--(N--R.sub.1 --N--R.sub.2 --aminomethyl)--4--hydroxystyrene (x=2,4,5 or 6, R.sub.1 =an alkyl group with C.sub.1-4, R.sub.2 =a substituent group corresponding to H(CHOH).sub.n CH.sub.2 --where n=1-7.
However, both the aluminum non-chromate surface treatment agent and treatment method disclosed in the above U.S. Pat. No. 5,089,064 and the metal treatment composition and treatment method disclosed in the above WO 9514539 fail to completely meet the aforesaid corrosion resistance requirements.